1. Field of the Invention
The invention generally relates to the processing of stereo imagery by using a Digital Photogrammetric Workstation (DPW), and more particularly, to a method of rigorously reshaping an overlapping pair of digital images through utilization of the workstation.
2. Description of the Prior Art
The human visual system perceives terrain depth when it fuses an overlapping pair of reshaped images, one seen in each eye, with parallax or the distance between common points (different projective geometry from overlapping reshaped image planes). When viewing digital imagery produced by a sensor, it is likewise important to reshape or rectify distorted overlapping images. Such reshaping removes geometrical distortions from the images to permit accurate visualization of objects in a three-dimensional coordinate frame, and prevents incorrect measurement of terrain elevations from the originally distorted images.
Conventionally, a polynomial rectification procedure (i.e., 6- or 8-parameter linear transformation) is utilized to geometrically transform an overlapping pair of originally distorted images into an overlapping pair of reshaped images. Polynomial rectification removes distortions by sampling the images so that they become uniform in scale and aligned for stereo viewing.
However, polynomial rectification does not completely correct the geometrical image distortions caused by interior and exterior orientations of the sensor, and by terrain relief displacements. Specifically, conventional polynomial rectification can only approximate the formula for a model that represents the physical characteristics of the sensor (optics, focal length, scan mechanisms, others) to produce computations for a ground-to-image transformation. As a result, the usefulness of the reshaped images produced by the conventional polynomial rectification process is limited because the approximations are geometrically imprecise.
A Digital Terrain Matrix (DTM) is a representation of the image data in the form of a two-dimensional array of elevation or vertical distance data points, generated from the stereo geometry of the reshaped imagery. The accuracy of a DTM produced from conventionally processed data is thus adversely affected by the limitations of such conventional processes.
Finally, conventional image rectification cannot mask the sensor source of the original images. As a result, the identity of the imagery source may be discovered by analysis of the sensor model associated with the original images. It is therefore undesirable to distribute rectified images when the identity of the sensor source is confidential.
FIG. 1 illustrates in greater detail the steps for performing conventional processing of an overlapping pair of originally distorted images in a Digital Photogrammetric Workstation (DPW). Specifically, the original images produced by a sensor and the associated sensor model are imported into a Digital Photo(rammetric Workstation (DPW) at step 10. Various known techniques are utilized to import the original images. The technique employed depends on the type of sensor model associated with the originally distorted images (e.g., Frame, Electro-Optical, Panoramic, LandSat, Satellite Platform Observing Terrain, Synthetic Aperture Radar). The DPW creates a corresponding support file that describes estimated sensor model parameters for the interior and exterior orientation for each of the original images. The images are then subjected to a triangulation in step 20 to adjust sensor parameters of interior and exterior orientation. Thereafter, the digital images are pair-wise rectified in step 30 to geometrically reshape the images into an overlapping pair of linear-rectified images thereby creating stereo imagery. The linear-rectified images then undergo stereo compilation in step 40 to generate a Digital Terrain Matrix (DTM) of vertical distances. Derivation of the vertical distances is achieved by measuring parallax or the distance between common points within each reshaped image, from different horizontal displacements caused by terrain elevation. Thereafter, a final orthophoto is created by using the DTM to remove the measured terrain elevation displacements. The described arrangement suffers from the differences noted above.